When radio transmissions are broadcast in free space, it is generally accepted that lower frequencies will propagate better than higher frequencies. However, when a radio signal propagates through a building or structure, other factors such as aperture sizes, boundary conditions and material constants come into play that may modify the propagation such that there may be a frequency that is optimal (in the minimum path loss sense) for a given structure that may not be the lowest frequency. For example, the propagation through buildings via waveguide effects may cause higher frequencies to propagate better than lower frequencies; an air handling system in a building may act as a waveguide below cutoff at VHF, but may duct a signal at 650 MHz out of the building with acceptable efficiency.
In a cognitive radio system, which may communicate within one of many frequency bands, a problem exists in how to choose the best frequency band for operation. As mentioned, simply choosing the lowest frequency band may be inadequate for communications within structures. Therefore, a need exists for a method and apparatus for channel selection that adequately chooses the minimum path loss frequency band for operation.